1st and 5th match - 3rd less than 1/5 - 7th less than 3rd. What really is the difference between 30% and 20%? Think like a kid sitting next to someone not standing in front of them. Sure, you can tweak but keep it simple. Equal to, more than, or less than has worked well for me. Plus, they can transfer those concepts to the sight-reading room.

Rather than a formula, remember the 7th is a dissonant pitch. So it one needs to sound like is going somewhere, even if it does not resolve. So to soft will sound like a wrong note. And the dissonance needs to a little softer than the root in this case.